1. Field of the Invention
The present invention relates to a parallel wiring and integrated circuit which use differential lines.
2. Description of the Related Art
LSIs which implement powerful multi-function devices by microfabrication and integration based on the scaling law as a leading principle are supporting signal processing of hardware in the current IT network-oriented society. A powerful processor has a clock frequency more than 1 GHz and a chip size on cm order. In one chip, a hundred million MOS transistors are integrated. The performance of an integrated circuit is determined not only by the characteristics of individual MOS transistors. The circuit performance is determined rather by the wiring technique for connecting individual transistors.
In a conventional LSI wiring design, a metal wiring line is expressed by an RC lumped constant circuit including resistors and capacitors. In recent years, however, the inductance components of wiring lines cannot be neglected as the LSI frequency becomes high. For this reason, it is becoming difficult in principle to design a long-distance wiring line as an RC lumped constant circuit.
Indeed, long-distance wiring lines determine the performance of a whole circuit. To reduce the wiring delay, introduction of a low-resistance metal Cu and a low-k interlayer dielectric film has been examined increasingly. In this idea, a long-distance wiring line is divided, and repeaters are inserted such that the wiring line can be handled as an RC lumped constant circuit. However, when the number of repeaters increases, the circuit area and power consumption also increase.
When the signal frequency is on GHz order, and the line length is on cm order, the inductance component of a wiring line cannot be neglected, and it cannot be handled as an RC lumped constant circuit. It is essential to regard signal transmission as electro-magnetic transmission and design a wiring line as a transmission line.
Generally, transmission lines can be classified into two types: an unbalanced transmission line including a signal line and ground, and a differential transmission line including two signal lines (there are also structures including ground). A differential transmission line has excellent crosstalk robustness because common mode noise can be canceled, unlike an unbalanced transmission line. Differential transmission line structures can be classified as follows on the basis of the difference in wiring structure.
(i) Stacked-Pair Line, (ii) Co-Planar Line, (iii) Microstrip Line, and (iv) Strip Line
Since a parallel wiring in an LSI includes a number of long-distance wiring lines in close vicinity, problems of wiring delay and crosstalk are posed. When a differential transmission line is used as a parallel wiring in an LSI, the problems of wiring delay and crosstalk can be solved. Presently, a parallel wiring on a board is implemented by arraying differential transmission lines in the horizontal direction. However, when a parallel wiring is to be designed in an LSI, many points must be taken into consideration in terms of wiring design, as compared to a board. For this reason, a parallel wiring design method unique to an LSI is necessary. For example, since the wiring size in an LSI is small, the ohmic loss component of wiring lines cannot be neglected. In addition, since the degree of freedom in wiring design is low, the limitation on the structure itself is large. If the inter-wire distance is shortened to increase the degree of integration, differential mode noise causes crosstalk between differential wiring lines in the parallel wiring. In a parallel wiring using differential transmission lines (i) to (iv), crosstalk by differential mode noise and the distance between differential wiring lines have a tradeoff relationship.
When a parallel wiring using differential transmission lines having the above-described wiring structure is designed in an LSI, the following problems are posed.    a) Crosstalk and the wiring area have a tradeoff relationship: (i) to (iv)    b) The characteristic impedance and the wiring resistance have a tradeoff relationship: (i), (iii), (iv)    c) When a pair of wiring lines are bent in the horizontal direction so as to prevent contact between them, they have different wiring lengths. Accordingly, mode conversion occurs (the differential component changes to an in-phase component): (ii) to (iv)    d) Since one layer is necessary as a ground plane, the wiring area and cost increase: (iii) and (iv)
As described above, when a parallel wiring is to be formed by using the conventional differential transmission lines, the problems of crosstalk, wiring area, characteristic impedance, loss, bending, and cost cannot be solved.